Epoxy resin systems that cure at ambient or low temperatures are highly desired by the coatings and adhesives industries. Such systems allow application and repair to occur under a wider range of conditions and extend the working season for many construction and repair operations.
The epoxy industry is in need of highly reactive amine curative compositions to enhance the rate of cure for the ambient and subambient curing of epoxy formulations for civil engineering, coating and paint applications. Polyacrylates are often incorporated into the epoxy formulation to enhance the rate of cure for ambient and subambient applications. Often mercaptans are used for very rapid cures in these formulations.
The use of alcohols and acids to accelerate the cure times of ambient and subambient cured epoxy formulations is well documented in the prior art. A good reference on this subject is Lee and Neville, "Handbook of Epoxy Resins," McGraw Hill (1967). Alcohols such as phenol, nonyl phenol, benzyl alcohol and furfurol are currently used by the epoxy industry to accelerate the curing process with commonly used amines and amine adducts. Although alcohols do accelerate the epoxy/amine reaction they can exude from the polymer detrimentally affecting the environment. As the alcohol migrates from the cured epoxy polymer the physical properties of that polymer change making them ineffective for the desired use. They also act as plasticizers to the epoxy matrix reducing the chemical resistance of the resulting cured epoxy polymer.
Other currently used accelerators include tertiary amines such as tri(dimethylaminomethyl)phenol and acids such as salicylic acid, toluenesulfonic acid and boron trifluoride. These accelerators, however, only moderately increase the cure rate of ambient and subambient cured epoxy formulations. They are fugitive, can effect the environment but do not plasticize the resulting cured epoxy formulation.
These approaches suffer from a variety of defects in that they utilize reagents that are either highly toxic, corrosive, or not wholly compatible with the rest of the epoxy formulation, thus affecting the final physical properties. A particular problem exists with the use of phenol as an accelerator, since it is highly corrosive to skin and is coming under increasing regulatory pressure.
In summary, alcohols and acids are limited in their effect on cure rate or on the physical or chemical properties of the resulting cured epoxy formulation.
Adducts from amines and mono- and diepoxides have long been used in the epoxy industry as curing agents and variants have been reported wherein excess amine is stripped from the product. The advantages of the formation of such adducts include lower volatility, reduced tendency to blush and exude, and less irritation potential. These adducts are also discussed at length in Lee and Neville, "Handbook of Epoxy Resins," McGraw Hill (1967).
JP01080423 and JP01080422 describe the preparation of a gas separation membrane by reacting equimolar amounts of an aliphatic or alicyclic diepoxide with a primary monoamine having two active hydrogens as a hardener. The resulting polymer exhibits selectivity for oxygen separation from air.
JP63148663 describes an epoxy resin composition comprised of a novolak phenol resin, a novolak epoxy resin and a primary monoamine compound used to seal a semiconductor device. The three reactants are blended in the proper equivalent amounts to generate a polymer upon heating.
JP60231723; JP62153317; JP61143419 and JP60231734 describe an additive, hydroxyalkyl amine, and the use of that additive in epoxy resin compositions. The additives are expressed by the following formula: ##STR2## (where R1, R2, R3, and R4 are each a hydrogen, a C1 to C17 saturated or unsaturated aliphatic, alicyclic, aromatic or heterocyclic group, or a heterocyclic residue from which the nitrogen to which R1, R2, R3, and R4 are bonded has been removed; each of these groups may substituted with a halogen group, a nitro group, an alkoxy group, an allyloxy group, or an acetyl group; and R1, R2, R3, and R4 may be the same or different). The stoichiometry between epoxy groups and NH groups used to prepare the hydroxy alkyl amine of this invention may be between 1/10 and 10/1, but it is preferable to use a range of 1/1.5 to 1.5/1. The patent teaches the use of these materials to increase the rigidity of an epoxy resin without markedly sacrificing the elongation. Both primary and secondary amines are employed in the teachings of this patent.
U.S. application Ser. No. 08/582,782, filed 4 Jan. 1996 entitled "Fast Cure Amines For Ambient And Subambient Cure Of Epoxy Resins", discloses an amine curative composition for epoxy resins consisting essentially of the reaction product of monomethylamine and a polyglycidyl ether reacted in a ratio of &gt;2 moles of monomethylamine per equivalent of epoxide and containing a maximum of 25 eq % tertiary amine.